Earlier studies from this laboratory demonstrated that the in vitro response of human T helper cells (Th) to HLA alloantigens is mediated by two distinct pathways for antigen presentation called the indirect and direct pathways, which respectively involve presentation of HLA antigens on self antigen presenting cells or direct presentation by the allogeneic stimulator cells. The laboratory further showed in renal transplant patients that the indirect pathway but not the direct pathway was correlated with kidney graft rejection. We have now begun to study rejection of cardiac allografts in mice and humans. In the murine heteopic heart transplant model involving immune suppression by cyclosporin A, we found a correlation between rejection and an intact indirect pathway but not between rejection and an intact direct pathway. It appeared that rejection was mediated by host-anti-donor cytotoxic T lymphocytes (CTL). In patients who received a cardiac transplant, biopsy-determined lymphoid infiltration (the clinical test for rejection) was correlated with an intact indirect pathway but not the direct pathway of allorecognition. These findings suggest that (similar to the results of renal allografts) host antigen-presenting cells, rather than antigen presenting cells resident in the graft, are mainly responsible for cardiac allograft rejection. These results also suggest that our test may provide an immunologic assay that detects rejection of human heart transplants.